Manufacture of electrical rectifiers



Sept. 6, 1938. p ow 2,129,410

MANUFACTURE OF ELECTRICAL RECTIFIERS Filed Oct. 23, 1955 INVENTOR PhiZzLpHDowZz'ng ma.

HIS ATTORNEY Patented Sept. 6, 1938 UNITED STATES PATENT OFFICE Philip H. Dowling, Forest Hills, Pa., assignor to The- Union Switch & Signal Company, Swissvale, Pa., a. corporatlonof Pennsylvania Application October 23, 1935, Serial No. 46,358

22 Claims.

My invention relates to the manufacture of electrical rectifiers, and particularly to the manufacture of rectifier elements for copper oxide rec- One object of my invention is to improve the rectifying, characteristics of the rectifier elements.

I will describe one process of manufacture embodying my invention, and will then point out the novel features thereof in claims.

In the accompanying drawing, Fig. 1 is a view showing in elevation one form of blank ready to be prepared as a rectifier element in accordance with one process of manufacture embodying my invention. Fig. 2 is a view showing a number of blanks assembled on a support as they appear during one step in the process of manufacture. Fig. 3 is a vertical sectional view showing, in an exaggerated form, a rectifier element as it appears in another step in the process of manufacture embodying my invention. Fig. 4 is a view, showing a completed rectifier element, constructed in accordance with my invention.

Similar reference characters refer to similar parts in all four views.

In the process of manufacturing copper oxide rectifier elements, as it is now generally practiced, a number of copper blanks which may, for example, be-similar to the blank A shown in Fig. 1, are first cleaned in any suitable manner, as by a sand blast, and these blanks are then assembled in pairs on a suitable support B in the manner shown in Fig. 2 so that the blanks of each pair have their adjacent faces A in contact. The blanks are then heated in the presence of air in an electric furnace, the temperature of which is usually maintained at about 1860 F. This heating of the blanks causes a layer of red or cuprous oxide to be formed on the blanks, and is continued until the cuprous oxide layer is of the desired thickness. After a sufficient amount of oxide has been formed on the copper blanks, these blanks are then transferred immediately to a second furnace which is maintained at a temperature of approximately 1050 F., and are allowed to remain in this latter furnace only for a sufficient length of time to permit them to cool down to the temperature of the second furnace. The oxidized blanks are then removed from the second furnace and are suddenly cooled or quenched as by plunging the blanks into cold water or a current of cold air. Each blank then appears as shown in Fig. 3 from which it will be seen that each blank A is now covered with an inner coating D of cuprous or red oxide of copper and a thin outer coating C of black oxide of copper. It will also be seen from an inspection of Fig. 3 that the flat face A of the plate which is exposed during the oxidizing process, that is to say, the lower face, has a much heavier oxide coating than the upper face. Each blank is next treated to remove the black oxide from the lower face of the blank, and both the black and red oxide from the upper face, after which the blank will appear as shown in Fig. 4. One process which is particularly suitable for removing the excess oxide from the blanks is described and claimed in my copending application for Letters Patent of the United States, Serial No. 749,078, filed on October 19, 1934, now a patent, No. 2,094,642 issued October 5, 1937. As a last step, the exposed face of the cuprous oxide remaining on the blank is carbonized, as by rubbing into this face powdered petrolium coke.

Rectifier elements of the type described exhibit the characteristic of offering a relatively low resistance to current flowing through the elements from the cuprous oxide to the copper and a relatively high resistance to the flow of current through the elements in the opposite direction. The resistance of the elements in the low resistance direction will hereinafter be referred to as the conducting resistance, while the resistance of the elements in the high resistance direction will hereinafter be referred to as the blocking resistance.

I have found that by modifying the process just described in the manner which I will now describe, the rectifying characteristics of the rectifier elements can be improved to a marked degree. This modification consists primarily in retaining the oxidized blanks in the second furnace for a longer period of time than is required to just cool these blanks to the quenching temperature. With the usual furnace equipment the time required to cool the blanks to the quenching temperature is about four minutes, and the rectifying characteristics of the elements for most practical purposes will continue to be improved as the time during which the oxidized blanks are retained at the quenching temperature increases, at least until this time exceeds two hours. The time during which the blanks are retained at the quenching temperature will, for convenience, be referred to hereinafter as the annealing time.

The principal effect of subjecting the oxidized blanks to an annealing time in the manner just described appears to be to reduce the resistance of the elements in the low resistance direction. This reduction in the resistance of the elements in the low resistance direction is accompanied by a decrease in the resistance of the elements in the high resistance direction. The amount of the decrease in either direction is also a function of the voltage which is impressed on the ele-' ments. The decrease in resistance of the elements in both the high resistance and the low resistance directions due to the annealing time appears to be a maximum at about .2 volt for relatively short annealing times. As the ennealing time increases, this point of maximum effects shifts to lower voltages in so far as the resistance of the elements in the low resistance direction is concerned, but appears to remain substantially constant in so far as the resistance of the elements in the high resistance direction is concerned. It is believed that this decrease in resistance of the elements due to the annealing time results from the absorption of excess oxygen by the cuprous oxide, and that the temperature at which the oxidizedblanks are maintained during this annealing time is particularly suitable for this purpose. One reason for believing that the decrease in resistance of the elements due to the annealing time is caused by the absorption of excess oxygen is that the results obtained are in line with present knowledge concerning the effect of excess omen on the resistivity of the cuprous oxide.

One of the most important characteristics of a rectifier element is its rectifying ratio, that is, the ratio of the current which fiows through the element in thelow resistance direction at a specified voltage to that which flows through the element in the high resistance direction at the same or someother specified voltage. and it is desirable that this ratio should be as high as possible. Under certain special operating conditions, the voltage which is impressed on the elements in both the low resistance and high resistance directions will be the same, but under most operating conditions, the voltage which is impressed ,on the elements in the high resistance direction will be several times higher than the voltage which is impressed on the elements in the low resistance direction. For example, the operating voltage which is generally impressed on the elements in the high resistance direction is about 4 volts, while the operating voltage which is generally impressed on the elements in the low resistance direction varies between .2 and .5 volt. With the elements which are subjected to an annealing time in the manner described, the rectifying ratio is improved by increasing the annealing time as long as the voltage to which the elements are subjected in the low resistance direction is less than .5 volt and the voltage to which the elements are subjected in the high resistance direction is about 4 volts. Within the usual operating range of voltages, this improvement becomes greater as the voltage in the low resistance direction decreases. For any given conducting direction voltage. although the rectifying ratio becomes greater the longer the annealing time, it shows a saturating tendency. That is to say, for any given voltage between .2 volt and .5 volt in the low resistance direction and about 4 volts in the high resistance direction, the rectifying ratio does not increase indefinitely with increases in the annealing time. When the voltage at which the elements are operated is the same in both the low resistance and the high resistance directions, the rectifying ratio first increases and then decreases with increases in the annealing time. It will be seen, therefore, that when the elements are operated at the same voltage in both the low resistance and the high resistance directions, an optimum annealing time exists above which the rectifying ratio will not be improved. For most practical purposes, however the annealing time that can be profitably em-' ployed, at least in so far as the rectifying ratio is concerned, is limited only to the extent that the added gains would not Justify the added expense.

In addition to decreasing the resistance of the elements in the low resistance direction and to improving the rectifying ratio, the annealing time appears to improve the uniformity of the resistance of the elements in the high resistance direction, and also to improve the aging characteristics of the elements.

I have also found that when the rectifier elements are subjected to an annealing time in the manner just described. a further improvement in the operating characteristics of the elements for certain conditions of operation can be effected by shortening the oxidation time of the blanks, that is to say, the time during which the blanks are retained in the first furnace. With the hereinbefore described process as employed prior to my present invention, the oxidation time, with a furnace having sufficient thermal capacity to bring the blanks up to the oxidation temperature in a relatively. short time, is usually between eight and thirteen minutes. If this time is decreased to five minutes, the annealing time remaining constant, the resistance of the elements in the high resistance direction will be about the same as if the elements had been oxidized for thirteen minutes. For oxidation times between thirteen minutes and-five minutes, the resistance of the elements in the high resistance direction appears to be lower at low operating voltages and higher at high operating voltages than that of elements which have been oxidized for either thirteen minutes or five minutes. The magnitude of the blocking resistances which are obtained for oxidation times between thirteen minutes and five minutes are somewhat erratic, but good blocking resistance characteristics can be obtained with short oxidation times coupled with relatively long annealing times.

With reference to the resistance of the elements in the low resistance direction for oxidation times less than thirteen minutes, the annealing time remaining constant, elements oxidized for eight minutes have consistently lower conducting resistances than those oxidized for thirteen minutes. When the oxidation time is reduced below eight minutes the result is obtained that for a given annealing time, the conducting resistance of the elements when the elements are operated at low voltages increases as the oxidation time decreases. For elements operated at higher voltages, the results are not as consistent but the reverse effect is obtained. That is to say, as the oxidation time decreases, the conducting resistance decreases. It will be seen, therefore, that for elements which are operated at lowvoltages in the conducting direction, there is an optimum oxidation time of about eight minutes at which the conducting resistance becomes a minimum, but for the higher operating voltages if there is an optimum oxidation time at which the conducting resista ce is a minimum, it appears to be .below five nutes.

It follows from the foregoing that for rectifier elements which are to be operated at known voltages in th high resistance and low resistance directions, it is possible, by properly choosing the ing the oxidized blank to a temperature of apoxidizing and annealing times, to manufacture the elements in a manner which will cause them to have the best possible operating characteristics for the intended purpose. It is obvious,

of course, that the actual lengths of time the blanks are maintained in the oxidation and annealing ovens in order to produce a given result will depend to some extent upon the relation of the capacities of the respective ovens to the total amount of copper contained in any one charge.

I have further found that forcertain applications of the rectifier elements, it is desirable to incorporate into the manufacturing process another heat treatment. This latter heat treatment consists in baking the elements at a relatively low temperature for a relatively long period of time at some time after the oxidized blanks have been quenched. The temperature at which the elements are baked is not critical and may, for example, be in the neighborhood of 212 F. The baking time is likewise not critical and may be in the neighborhood of 16 to 72 hours. It should be noted, however, that as the baking temperature is decreased, the baking time should be increased. The baking operation will usually be performed after the elements have been assembled into a rectifying unit, but it can, if desired, be performed immediately after the quenching operation.

One eifect of the low temperature bake is to decrease the tendency of the elements to break down and short-circuit when subjected to high voltage surges in the high resistance direction. Another effect of the low temperature bake is to improve the aging characteristics of the elements. It should be particularly pointed out that baking the elements produces a beneficial effect on the elements whether or not the elements are subjected to an annealing time.

One advantage of the manufacturing process embodying my present invention is that elements which are manufactured by this process operate at higher eificiencies and lower temperatures than it has heretofore been possible to obtain, thus making it possible to increase their output at a given temperature or to obtain a given output at a lower temperature, and thereby decrease the aging of the rectifier. The high efficiency of the elements is also useful in connection with meter rectifiers, and the low resistance is useful in such applications as the snubbing of relays, or the absorption of the inductive discharge from a magnet when its circuit is opened.

Although I have herein shown and described only one process of manufacture embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In the process of preparing a rectifier element which consists in oxidizing a copper blank at a temperature of approximately 1860" F., cooling the oxidized blank to a temperature of approximately 1050 F., and then quenching the oxidized blank while the blank is at the 'last mentioned temperature, the step which consists in retaining the oxidized blank at said last mentioned temperature for a period of time suflicient to materially decrease the resistance of the element in the low resistance direction.

2. In the process of preparing a rectifier element which consists in oxidizing a copper blank at a temperature of approximately 1860 F., coolproximately 1050 F.. and then quenching the oxidized blank while the blank is at the last mentioned temperature, the step which consists in retaining the oxidized blanks at said last mentioned temperature for a period of time sufficient to materially improve the rectifying ratio of the element.

3. In the process of preparing rectifier elements which consists in oxidizing copper blanks at a temperature of approximately 1860 F., cooling the oxidized blanks to a temperature of approximately 1050 F., and then quenching the oxidized blanks whilethe blanks are at the last mentioned temperature, the step which consists in retaining the oxidized blanks at said last mentioned temperature for a few minutes to improve the uniformity of the resistance of the elements in the low resistance direction.

4. In the process of preparing a rectifier element which consists in oxidizing a copper blank at a temperature of approximately 1860 F., cooling the oxidized blank to a temperature of approximately 1050 F., and then quenching the oxidized blank while the blank is at the last mentioned temperature, the step which consists in retaining the oxidized blank at said last mentioned temperature for a period of time suflicient to materially improve the aging characteristics of the element.

5. In the process of preparing a rectifier element which consists in oxidizing a copper blank at a temperature of approximately 1860 F., cooling the oxidized blank to a temperature of approximately 1050 F., and then quenching the oxidized blank while the .blank is at the last mentioned temperature, the step which consists in retaining the oxidized blank at said last mentioned temperature for a substantial period of time the duration of which depends upon the voltages at which the element is to be operated in both the high resistance and low resistance directions;

6. In the process of preparing a rectifier element which consists in oxidizing a copper blank at a temperature of approximately 1860 F., cooling the oxidized blank to a temperature of approximately 1050 F., and then quenchingthe oxidized blank, the step which consists in retaining the oxidized blank at the quenching temperature for a predetermined period of time, said predetermined period of time being so chosen that for a particular operating voltage the resistance of the element in the low resistance direction will be a minimum.

7. In the process of preparing a rectifier element which consists in oxidizing a copper blank at a temperature of approximately 1860 F., cooling the oxidized blank to a temperature of approximately 1050 F., and then quenching the oxidized blank, the step which consists in retaining the oxidized blank at the quenching temperature for a predetermined period of time, said predetermined period of time being of such duration that the rectifying ratio of said element will be a maximum for. a particular operating voltage in both thelow resistance and high resistance directions.

8. In the process of preparing a rectifier element which consists in oxidizing a copper blank at a temperature of approximately 1860 F., cooling the oxidized blank to a temperature of approximately 1050 F., and then quenching the oxidized blank, the step which consists in retaining the oxidized blank at the quenching temperature for a predetermined-period '01 time, said predetermined period oi! time being of such duration that the rectifying ratio of said element will be a maximum when the element is subjected to one voltage in the high resistance direction and another voltage in the low resistance direction.

9. The process of preparing a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at a temperature of approximately 1860 F. for a predetermined period of time to form a coating of cuprous oxide on the blank, cooling the oxidized blank to a temperature of approximately 1050 F., subsequently retaining the oxidized blank at said last mentioned temperature for a predetermined period of time, and then quenching said oxidized blank, said predetermined period of time being so chosen that said element will have a minimum resistance in the low resistance direction for a particular operating voltage;

10. The process of preparing a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at a temperature of approximately 1860 F. for a predetermined period of time to form a coating of cuprous oxide on the blank, cooling the oxidized blank to a temperature of approximately 1050 F.,subsequently retaining the oxidized blank'at said lastmentioned temperature for a predetermined period of time, and then quenching said oxidized blank, said predetermined period of time being so chosen that said element will have a .maximum rectifying ratio for a particular operating voltage.

11. The process of preparing a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at an oxidizing temperature to form a coating of cuprous oxide on the blank, cooling the oxidized blank to a temperature of approximately 1050 F., subsequently maintaining said blank at said last-mentioned temperature for a period of time ranging from a few minutes to a maximum of two hours, and then immediately quenching said blank.

12. The process of preparing a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at a temperature of approximately 1860 F. for a period of between approximately five minutes and approximately thirteen minutes to form a coating of cuprous oxide on said blank, cooling the oxidized blank to a temperature of approximately 1050 F., subsequently maintaining said blank at said lastmentioned temperature for a period of time ranging from a few minutes to a miximum of two hours, and then immediately quenching said blank.

13. The process of preparing a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at an oxidizing temperature to form a coating of cuprous oxide on said blank, cooling said blank to a temperature at which cuprous'oxide absorbs .excess oxygen, subsequently maintaining said blank at said lastmentioned temperature for a pretermined period of time, quenching said oxidizedblank to rapidly cool it, and finally baking said blank at a temperature of approximately 212 F.

14. The process of preparing a rectifier element which consists in heating a copper blank on the blank, cooling the oxidized blank to a temperature of approximately 1050 F., subsequently retaining the oxidized blank at said lastmentioned temperature for a predetermined period of time, quenching said oxidized blank to cool it, and then baking said blank at a relatively low temperature.

15. The process of preparing a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at a temperature of approximately 1860 F. for a predetermined period of time to form a coating of cuprous oxide on the blank, cooling the oxidized blank to a temperature of approximately 1050 F., subsequently retaining the oxidized blank at said last mentioned temperature for a predetermined period of time, quenching said oxidized blank in cold water tocool it, and then baking said blank at a temperature of approximately 212 F.

16. The process of preparing a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at a temperature of approximately 1860 F. to form a coating of cuprous oxide on said blank, cooling the oxidized blank to a temperature of approximately 1050 F., quenching-said blank to further cool it, and then baking said blank at a relatively low temperature.

17. The process of preparing a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at a temperature of approximately 1860 F. to form a coating of cuprous oxide on' said blank, cooling the oxidized blank to a temperature of approximately 1050" F., quenching said blank to further cool it, and then baking said blank at a temperature of approximately 212 F. to decrease the tendency of said element to break down due to high voltage surges.

18. The process of preparin'g a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at a temperature of approximately 1860 F. for a period of between approximately five minutes and approximately thirteen minutes to form a coating of cuprous oxide on said blank, cooling the oxidized blank to a temperature of approximately 1050 F., subsequently maintaining said blank at said lastmentioned temperature for a period of time ranging to a maximum of two hours to anneal said blank, and then baking said blank at a relatively low temperature to decrease the tendency of the rectifier element to break down due to high voltage surges in the high resistance direction.

19. The process of preparing a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at a temperature or approximately 1860 F. for a period of be-' tween approximately five'minutes and approximately thirteen minutes to form a coating of cuprous oxide on said blank, cooling the" oxidized blank to a temperature of approximately 1050- F., subsequently maintaining said blank at said last-mentioned temperature for a period of time ranging to .a maximum of two hours to anneal said blank, and then baking said blank at a temperature of approximately 212 F. for a period of time between approximately 16 and approximately 72 hours. I l

20. The process of preparing arectifier element which consists in heating acopper blank ly maintaining said blank at said last-mentioned Ill 21. The process of preparing a rectifier element which consists in heating a copper blank in an oxidizing atmosphere at an oxidizing temperature to form a coating of cuprous oxide on the blank, cooling the oxidized blank to a temperature of approximately 1050" F., subsequently maintaining said blank at said last-mentioned temperature for several minutes, and then immediately quenching said blank.

22. The process of manufacturing rectiflers which consists in heating copper blanks in an oxidizing atmosphere at a temperature of approximately 1860 F. to form a coating of cuprous oxide on said blanks, cooling the oxidized blanks to a temperature of approximately] 1050* F., quenching said blanks to further cool them,

assembling said blanks into a stack, and then 10 baking the stack at a relatively low temperature.

PHlHP H. DOWLING. 

